He’s been a hero to liberals for voting against big banks, the Iraq war and the Patriot Act.

But Sen. Russ Feingold’s decision to become the only Democratic vote against Wall Street reform has left many questioning his strategy.

Progressives figured the Wisconsin Democrat would be first in line to sign on to the financial regulatory overhaul. Instead, he’s been stubbornly, defiantly opposed to the legislation — Feingold calls it a cave-in to Wall Street — almost single-handedly delaying the final vote and denying, at least temporarily, President Barack Obama’s second legislative triumph.

But what really galls some on the left is that Feingold’s resistance opened the door for deal making — and Massachusetts Republican Sen. Scott Brown walked right through, making Brown the kingmaker on the bill that many on the left thought Feingold could have been.

“If he had said loudly and clearly during recent conference negotiations that he’d vote yes if, and only if, the strongest version of the Lincoln proposal and ‘Volcker rule’ were in the final bill, it would have made Scott Brown irrelevant and dramatically changed the negotiations,” said Adam Green, co-founder of the Progressive Change Campaign Committee, referring to Sen. Blanche Lincoln’s derivatives crackdown and former Fed chief Paul Volcker’s limits on risky bank investments.

“Instead, he remained silent, let the bill get watered down and then spoke out after the negotiations were over. That’s not bold; it’s weak and unstrategic — and it helped Wall Street rip off the public to the tune of billions in the future,” Green said.

Brown’s sway over the bill was illustrated again Monday when he announced he would vote yes, allowing Democrats to finally get the 60 votes they need to pass the landmark legislation — but not before Democrats watered down key parts of the bill to Brown’s liking, including allowing banks to do a small bit of trading with their own funds.

Sen. Olympia Snowe (R-Maine) also announced that she, too, would support the bill — potentially clearing the way for a vote as early as this week.

In an interview Monday, Feingold remained unapologetic and took issue with claims that he didn’t do enough.

At the outset of the floor debate, he detailed his test for this legislation: whether it would prevent another financial crisis. He co-sponsored five amendments that he said would have addressed his issues, namely restoring Glass-Steagall Act firewalls between commercial and investment banking and restricting banks from becoming so big that they threaten the financial system.

And Feingold said that he spoke several times with administration officials, Senate leaders and members of the conference committee, but at no time did they show any interest in moving closer to his position. Once it became clear that his two biggest issues had no shot in the Senate, he said he couldn’t get to “yes.”

Feingold said criticism of him as less than involved is generated by those within the “axis between Wall Street and Washington, and I’m not part of that axis.”

“I don’t know what they are talking about,” Feingold said. “Why would I try to work with people who are trying to kill the things that I think are most important? They were asking me to cut a deal that was against the interests of the American people and the people of Wisconsin.”

To be sure, some progressives cheered Feingold for stating what one called “inconvenient truths” about the legislation. And Feingold is known as a contrarian who has bucked his party at times. Even if he had engaged more vigorously in the negotiations, his demands might have alienated moderates.

But his opposition to the Wall Street reform legislation struck some of his allies as a step too far and caught off-guard even those well acquainted with his against-the-grain voting patterns.

“There wasn’t a sense of it early on that he should be one of the targets,” said Robert Kraig, executive director of Citizen Action of Wisconsin and longtime observer of Feingold. “I really don’t think he developed into a Tier One target where people thought he was a problem until very late in the process. On the national level, they were assuming he would come around.”

He never did — and now, he has come to embody Democrats’ divided emotions over the bill. “A lot of us want to love Russ Feingold, but he consistently shows lackluster ability to use smart leverage in important policy fights like health care and Wall Street reform,” Green said.

The Wall Street bill reinforced Feingold’s standing as a Washington irritant, but back in Wisconsin, he’s banking on that reputation to pull him through what is turning into a tough reelection.

Feingold released a 60-second radio ad Tuesday, positioning himself as a scourge of special interests and party leaders.

“I continue to be an independent voice who is tough on government spending and will stand up to both political parties,” he said in the commercial. “Maybe I won’t make a lot of new friends, but at least you will have one fighting for you and future generations.”

One Democratic Senate aide said Feingold should have made more of a public push to fix the bill. Sure, he signed on to the amendments most wanted by liberals, but he didn’t put any work into building support for them, the aide said.

Because of his lower profile, some Democrats were shocked when he was one of only two Democrats to oppose moving the bill to final passage in May after three weeks of debate.

His criticism irked colleagues, who objected to his claim that the bill failed to address the threat posed by “too big to fail” banks. Annoyance with Feingold came to a head during one of the Senate votes, when Sen. Mark Warner (D-Va.) confronted him on the floor, setting off an animated discussion over his public statements.

“He very much wanted me to accept the idea that this bill was significant on the issues I care about,” Feingold said. “I said it didn’t get to the core of the problem.”

Green said his group offered “many times” to meet with Feingold aides, to no avail, creating a stark contrast with the relationship forged with Democratic Sens. Carl Levin of Michigan and Jeff Merkley of Oregon, who led efforts to strengthen the Volcker rule.

As someone who has opposed interstate banking, the bank deregulation in 1999 and the 2008 bank bailout, Feingold said, “I don’t really have to put up with people who say I haven’t been involved in this.”

Kraig, the Wisconsin operative, said Feingold was being Feingold. His approach to the bill carried the hallmarks of his leadership style: He doesn’t take up the role of backroom deal maker. Instead, he makes an assessment about a bill on its merits, and if lawmakers want to get his vote, they have to meet his standards.

“It’s not like he is constantly voting against something that is not perfect,” Kraig said. “But when he does make a decision and digs in, he is hard to persuade.”